Coordinate measuring machines are known of the type having a bridge mounted on a base for guided movement along a first horizontal axis, a carriage mounted on the bridge for guided movement along a second horizontal axis orthogonal to the first axis, and a probe mounted for vertical movement on the carriage. The probe tip is thus movable along three orthogonal axes above the surface of a table mounted to the base and adapted to support an object to be measured.
The carriage is mounted for horizontal movement on an upper crossmember of the bridge. In this design, the bridge includes a pair of upstanding members disposed on either side of the table supporting the upper cross member, and also has a lower cross member extending beneath the table joining the lower ends of the upstanding members. The bridge is thus formed in a ring shape, which is of improved rigidity over open leg constructions.
The bridge moves on the base along precisely aligned ways formed on the base. Movement of the probe over points on the object to be measured allows dimensional measurements to be made on objects on the table by transducers which measure the extent of movement along each axis, and which generate corresponding electrical signals, which are processed and displayed.
The ring bridge type of coordinate measuring machine is disclosed in U.S. Pat. No. 4,594,791, issued on June 17, 1986, for "A Bridge Type Coordinate Measuring Machine", and also in USSR Pat. No. 621,955 issued in 1978.
In the aforementioned U.S. patents, the bridge is mounted on the ways by air bearings, which require very smooth way surfaces for completely true repeatable travel of the bridge on the ways.
Such machines also incorporate optical transducers as of the type shown in U.S. Pat. No. 2,886,717 issued on May 12, 1959 for "Measuring Apparatus" and U.S. Pat. No. 2,886,718, issue on May 12, 1959 for "Measuring Apparatus" and are operated in industrial settings, such that it is desirable that covering of the interior spaces and ways be provided to keep out dust and dirt and other contaminants.
Since the bridge moves relative the stationary table and base, any covering must accommodate this movement while not resulting in large openings that would allow entry of contaminants.
In such machines, deflection of the parts is advantageously kept to a minimum, since any deflections may produce slight non-repeatable discrepancies between actual probe movement and movement of the bridge and carriage as read by the transducers.
Bellows or accordion type shields have been employed in such coordinate measuring machines to cover the transducer components, which are compressed by the movement of the bridge. Such compression causes forces to be developed which tend to cause undesirable deflections of the machine structure. See U.S. Pat. No. 3,749,501 issued on July 31, 1973 for a "Measuring Machine for an example of such shielding.
Movement of the bridge may be powered as under computer or joystick control for more rapid measuring, and in such instances are powered movement of a bridge having portions extending below the table and alongside the base creates the possibility of inadvertent contact with a person who moves into the path of the bridge movement alongside the table. It is an object of this invention to lessen the chances of such contact by the design of the machine.